The Vasa Museum receives over million visitors each year. (Photo: Nalle Westman)

By NALLE WESTMAN

Stockholm’s top attraction, the 17th century warship Vasa, is an unique experience. It’s the oldest intact ship in the world and also the largest waterlogged wooden structure conserved ever. Nearly 30 million people has visited the ship since November 1961, when the provisional museum Wasa Shipyard opened. The millions of visitors expose the ship to a great stress. No matter how intensive care it’s under, the ship won’t last forever. The scientists are fighting a high-tech battle against the odds.

In 1961 Vasa was raised from the bottom of Stockholm harbor after lying there 333 years. Without a proper conservation the ship would have quickly deteriorated if the hull had been simply allowed to dry. The conservation was done by spraying polyethylene glycol (PEG) over the ship. PEG was used as a substance to replace the water molecules in the wood structure, thereby preventing the cracking and shrinking. The spray treatment lasted 17 years, followed by another 9 years slow air-drying.

The most ideal way to preserve Vasa would be placing it in oxygen-free dark room, but this is out of the question; the museum generates hefty profits through admissions, retail sales and a restaurant. Besides, what good would it be to have a national monument that nobody can see. The museum is constantly monitoring the ship for damage caused by decay or warping of the wood. In 2004 a new climate system was installed to stabilize the temperature and humidity. The main hall is kept at a temperature of 18–20 °C (64–68 °F) and a humidity level of 55%.

In 2002 spots of white residue were noticed on Vasa. These stains were the first indications that the ship contained considerable amounts of sulphuric acid. It is estimated that the wood now contains around two tons of sulphuric acid, and a further five tons or so may build up once all the sulphur has oxidised; this might eventually destroy the ship almost entirely. Another threat seems to be that treating wood with PEG in an acidic environment can generate formic acid and eventually liquify the wood. Also a current problem is that the old oak of which the ship is built is starting to give way. Even though the odds are against it, the scientists keep on fighting to preserve Vasa as long as they can.